Structural panels of comparatively light but rigid construction are known in which a low density core, generally of honey-combed or other cellular type material, is bonded at its upstanding edges defining the cells, to a face sheet on one or both sides of the core. Such structural panels have found wide use, for example, in the construction of aircraft wings, which undergo considerable stress in normal use. It is therefore important that a secure adhering bond be maintained between the face sheet and the core substantially over the entire area of the panel. Testing of the panels for locating bonding defects has at times been carried out manually by light tapping of local areas with a hammer for differences in sounding indicating to the experienced ear of the operator a loose area or defective bond.
It has been proposed to improve the described manual testing by applying to local areas of the panel an inverted bowl member associated with means for producing a vacuum between the inner surface of the bowl and the surface of the panel, so that the applied suction would lift the skin or face sheet of the panel at places where it was not properly bonded to the core. Means were provided for detecting any bulged areas. Such arrangement is disclosed, for example, in U.S. Pat. No. 2,694,924. Since the operation of this type of device necessitated that for effecting the desired suction, a hermetic seal be maintained at the rim of the bowl which defined the area being tested, considerable time was involved in making and then breaking the vacuum to enable the test apparatus to be moved over other areas of the panel to be tested.
Other types of transducer testing devices are known, such as those utilizing ultrasonic or other fault detecting means, which require elaborate systems for analyzing the received signal and for transforming the same to useful intelligence for evaluation, and requiring special skills for their operation. Most of the known transducer test instruments of these types depend upon the use of an impedance coupling liquid. Also, those that can be moved over a surface for scanning require the use of lubricants to reduce surface friction.